1. Field of the Invention
This invention relates generally to the use of optical devices to sense the progress of processes, transmit signals related to such progress, detect such signals and extract from such signals information to control such processes. More particularly, this invention relates to the removal of interferences from optical emission signals during endpoint determination in dry etching processes for the fabrication of micro-electronic devices, including, but not limited to, semiconductor devices, and for other micro-machining processes, which are accompanied by the emission of light from the reactants, products, film being etched or some combination thereof.
2. Brief Description of the Prior Art
Photolithography makes it possible to transfer a desired circuit pattern to a surface of a semi-conductor device. This is commonly done by the application of a photoresist film to a wafer followed by imaging and developing processes which form the desired pattern upon the wafer.
The wafer is then etched in the pattern formed by the developing processes. Dry etching, which is the chemical processing of microcircuit films in a low pressure reactor, is achieved by a variety of dry etch processes, including, but not limited to, plasma etch, reactive ion etch, ion milling, reactive ion beam etch, magnetron etch, and downstream etch. Gases, such as Carbon Tetrafluoride and others are fed into a plasma reactor and a high frequency discharge is used to convert the gases into reactive ions and molecular fragments which react with the film being etched.
During the etching steps it is important to monitor the progress of the etch and to detect the point at which the underlying material or film being etched is reached. This point is called the endpoint although the process may continue for a period after endpoint, which is referred to as overetching.
Optical emission spectroscopy is a current method used to detect process endpoint in plasma etching systems. This is possible because the plasma excites certain molecular species and causes them to emit light of wavelengths that are characteristic of each species being etched. In an optical monitoring system specific wavelengths of the light emitted from the plasma are selected and fed to detectors, such as photodiodes, photomultipliers, and array detectors which convert the light intensities into electrical signals. It is known that the intensity of the detected raw signals is related to the level of light detected and by selecting wavelengths which correlate to the reaction products of the particular process, the process may be monitored either at specific wavelengths are at all wavelengths by a spectral scan. In particular, by selecting a wavelength which corresponds to the emissions generated by the layer below the layer which is being etched, the point at which that layer is reached may be easily detected. When the film being etched has completely cleared from the underlying material or film, there is a chemical change both in the gas phase and on the film. Product species from the film are no longer being generated, and some reactants increase because they are no longer being consumed by the reaction. These chemical changes show up as changes in optical emission intensities. Thus by continuously monitoring the intensity of an appropriate emission feature, either a reactant or product of the etch reaction, a change in emission intensity generally signals removal of the film being etched and contact of the etching agent with the underlying material or film, or endpoint. The change in emission intensity which signals endpoint may either be an increase for a reactant emission or a decrease for a product emission or the presence of another reactant emission.
However, in some processes, the change in the optical emission signal being monitored for endpoint is small and difficult to detect. Where the signal or signal change at endpoint is small, the presence of interfering signals may obscure the signal or signal change and prevent the observation of endpoint or cause false endpoints to be read. Interferences may result from a number of sources. These include, but are not limited to the following:
(1) process related interferences which naturally occur in the chemical stirring of the plasma, and which may be manifested in plasma fluctuations and signal drift;
(2) Equipment created variations, which result from the actions of the operator in controlling the process, such as control loops and similar activities which are inherent in the normal operation of etching machines to stabilize flow, pressure, power, temperature or other variables;
(3) The practice in certain plasma etch machines of introducing a periodic variation in plasma density via an external magnetic field, or by modulating the RF power into the plasma or other means. These machines produce a low frequency, typically 0.1 to 100 HZ, periodic modulation of the plasma in the etch chamber.
In plasma etch machines such modulation may show up as a periodic oscillation in the optical emission signal and is often of a magnitude large enough to overwhelm the relatively small emission signal change at endpoint. Another obstacle to detecting endpoint in such systems or to perform plasma or process diagnostics based on the measuring of optical emission intensities is the superimposition of the oscillation on spectral scans, i.e. signal vs. wavelength, of the plasma emission.
(4). Plasma emissions from other species at or near the wavelength being monitored.
All of the interferences described above are frequently of a broad-band nature, i.e. occur at a variety of spectral wavelengths. To the extent this is true, optical emission intensities measured at two separate wavelengths will be partially correlated. There will be intensity fluctuations that are common to both wavelengths, because their cause is a common perturbation of the plasma. This correlation provides a vehicle for interference removal by simultaneous observation of two or more wavelengths and suitable combinations of the emission intensities.
The principal object of this invention is a method to monitor etch processes and to determine endpoints in plasma etch chambers in the presence of interferences which tend to obscure the signals which indicate the changes which are to be detected and observed.
Another object of this invention is to monitor processes and to determine endpoints in plasma in the presence of a periodically varying low frequency plasma modulation.
Yet another object of this invention is a method and device to detect endpoints using optical emission spectroscopy in systems where the optical emission signal at endpoint is small relative to the signal caused by plasma fluctuation.
Still another object of this invention is a method and device for analyzing the signals emitted at different wavelengths from a plasma emissions and isolating those portions of the signals which are correlated as contrasted to those which are uncorrelated.
An additional object of this invention is to monitor processes in the presence of emission interferences caused by other species which omit at or near the wavelength being monitored.
Optical emission spectroscopy is based upon detecting and observing emission intensity changes at selected wavelengths. Ideally, changes in emission intensity would result from single isolated source, however, in reality a number of sources contribute to such changes. These may be broadly characterized as follows:
(1) Random noise, comprising both electronic noise, which is generally quite low in well-designed instruments, and shot noise, which is inherent in the process of converting a light signal to an electrical signal;
(2) Correlated fluctuations, including intensity drift, i.e., intensity changes which occur simultaneously at two or more wavelengths, as discussed above;
(3) Periodic modulations of the plasma;
(4) Intensity changes arising from the removal of a film layer from the wafer or substrate surface i.e. endpoint.
The changes resulting at endpoint are what we desire to unequivocally determine, but such changes are generally obscured by the interferences described in (1), (2) and (3) above.
In the practice of this invention, the electronic portion of random noise is removed or minimized by good electrical design and the shot noise portion is minimized by collecting a large amount of light from the etch chamber.
The intensity changes resulting from correlated fluctuations are removed or minimized by observing two or more wavelengths simultaneously, which experience such fluctuations, but only some of which experience the endpoint intensity changes. An algorithm which suitably combines the signal intensities from both channels, e.g., a simple subtraction of Channel B from Channel A, will remove the common or correlated intensity fluctuations leaving only the endpoint intensity change, which we detect using existing endpoint detection algorithms.
Degenkolb et al U.S. Patent, 4,312,732, Method for the Optical Monitoring of Plasma Discharge Processing Operations and the references cited therein teach the basic concepts of optical monitoring of plasma discharge.